Dry gas seals are specified in the majority of new centrifugal compressors, yet many installed units are still equipped with conventional oil sealing systems. The benefits of dry gas seals are such that seal conversion from traditional oil seals to dry gas seals may be advantageous to compressor operators. However, end users should ask themselves several questions before deciding to retrofit their compressors with dry gas seals. The decision to retrofit a compressor with these upgraded seals may be dictated by economic factors, HSE constraints or technical considerations. Users should consider all of these factors when deciding whether or not to upgrade a compressor with dry gas seals.
In addition, the following precautions should be taken during project execution to ensure successful conversion: perform a detailed physical integration analysis of the dry gas seal in the existing compressor; conduct a detailed rotor dynamic analysis; select the proper gas seal system design for the compressor; and plan for operator training.
This article will discuss the factors end-users should consider before upgrading to dry gas seals, and the steps that should be taken to ensure a successful conversion once the decision is made to retrofit a compressor with dry gas seals.
Dry gas seals
To expect a totally leak-free sealing system between two parts in relative movement is unrealistic (e.g., between a static and a rotating part; between a housing and a shaft; in pumps, thermal motors, etc.). There are, however, efficient devices that may limit leaks, friction, and wearing at the interface of the moving parts.
Gas seals are among the most efficient means to minimize process gas leakage to the atmosphere and to reduce wear and friction.
The gas seal is also a reliable means to route effluent leaks to safe areas. Overall, the whole gas compression process benefits from the dry gas seal system.
Figure 1 shows the location of the seals in a typical centrifugal compressor. Their location is quite strategic, as they are the interface between the inside of the compressor (gas process at high pressure, high temperature) and the atmosphere (air and oil mist from the bearing cavity).
Due to the balance line, the gas seal only has to deal with the intake pressure of the compressor.
As will be explained later, the gas seal requires a high-quality gas to operate. Therefore, instead of using the gas present in the balance line, the seals are fed with a clean and dry gas, typically taken at the discharge of the compressor.
This gas is dried, filtered, heated (if necessary), and its pressure lowered to slightly above the intake pressure before being injected at the primary port of the seal.
The gas-seal principle is simple (Figure 2). The leakage (process gas) must be routed to a safe area; therefore, the leakage is forced to pass between a static and a rotating part. The rotating part is a grooved ring driven by the compressor shaft. The static part is a ring facing the rotating ring (but with only light axial movement).
When rotating, the grooves generate an aerodynamic effect that creates a gap (from 4 microns to 10 microns) between the rotating and stationary rings. The flow generated by the pressure differential leaks between the two faces, and then this gas leakage is routed to the venting system of the machine (flared) or vented.
Because of the gas film between the faces, this constant gap between them prevents the parts from rubbing against each other and makes the gas seal a contact-free device.
Gas seal arrangements
- A tandem gas seal is typically used for non-hazardous gases. In this arrangement, the sealing gas is injected at a pressure slightly above the intake pressure, so that a vast majority (over 80 percent) of it passes under the inner labyrinth teeth. The remainder (less than 20 percent) passes through the gap created by the lift-off effect and leaks to the flare (18 percent). The last sealing gas residues (2 percent) leak through the secondary stage to the vent.The other important device in the compressor seal is the tertiary (or separation) seal, which may be a labyrinth or segmented carbon rings. Its function is to prevent the bearing oil mist from migrating to the seal and the sealing gas from migrating to the bearing oil. This separation is made by a gas leak, which prevents the oil from entering the gas seal area on the inboard side, and also prevents the sealing gas coming from the secondary stage of the seal from polluting the bearing oil.
So, depending on the nature of the separation gas, the gas seal vent may vent a mixture of sealing gas (hydrocarbon) and nitrogen, or a mixture of sealing gas and air.
- A tandem gas seal with an intermediate labyrinth is used when the process gas is hazardous, e.g., lethal gas, flammable gas, or when it condensates at the primary seal outlet (Figure 3). In this scenario, a buffer gas, such as nitrogen, sweet gas or fuel gas, is required.The principle is the same as in the tandem gas seal with the addition of an intermediate labyrinth fed with an intermediate (sometimes called a buffer) gas, generally nitrogen. This prevents hazardous seal gas from leaking into the atmosphere.
- A double (-opposed or back-to-back) gas seal is used when the process gas is dirty, or when the sealing pressure is close to atmospheric pressure. A sealing gas (typically auxiliary gas) is needed, such as nitrogen, sweet gas or fuel gas.The configuration consists of two sealing faces (rotating ring and static seats) in a back-to-back arrangement. A primary advantage of this seal type is the lower number of ports required—one for the sealing gas; one for the vent; one for the separation gas; and one for the buffer gas (optional).
Because of the pressure differential between the inboard side of the gas seal and the sealing-gas port, and between the vent and the sealing-gas port, the flow is not symmetrical (a majority of the sealing gas enters the machine).
Generally in low-pressure applications, the available process gas pressure is not suitable to feed the gas seal, so an alternate source must be considered (e.g., nitrogen, fuel gas).
The nature of the sealing gas must also be compatible with the nature of the process; the alternate source could trigger unwanted chemical reactions or damage the downstream catalyst.
Why convert wet seals to dry gas seals?
- The number one reason for retrofitting conventional wet seals to dry gas seals is reliability. Dry gas seals are non-contacting mechanical seals, which eliminates the issue of seal wear. Theoretical lifetime is limited only by the secondary sealing elements (usually o-rings or polymer-based seals) whose lifespan can be as long as 15 years. It is not uncommon to see dry gas seals operating for more than 10 years before being refurbished, which is much longer than is expected for oil seals.Not only is the seal itself more reliable; so is the whole sealing system, because it is made of static components. Oil seal systems, on the other hand, have more components, including rotating machines (pumps, motors/turbines) and are more often prone to unscheduled maintenance.
- Local (or company-wide) HSE Regulations: Elimination of oil contamination by process gas has a positive environmental impact, since sour oil needs to be treated, stored, and disposed of. Sour seal oil treatment and disposal also has a significant cost.In terms of safety, disposing of contaminated oil removes a hazard of explosive mixtures in the oil reservoir of seal (and lube) systems.
- Reduced Operating Costs: Energy costs drop significantly, since seal oil pumps and degassing tank heating systems are not required when using dry gas seals. Power losses due to shear forces in gas seals are much lower than losses experienced in oil seals, which results in energy savings as well.
- Reduced Maintenance Costs: As stated above, the simplicity of gas seal systems means routine maintenance is less frequent and less costly than it is with oil seal systems.
- Reduced Emissions: Wet seal gas leakages are reduced 10-fold with gas seals, credited to the very thin running gaps between the seal faces. This results in cost savings for the end-user and reduced penalties on taxable gas flaring.
- Process Quality: Contamination of process gas by seal oil is eliminated, enabling higher quality process gas. Costs related to oil removal from process gas are also eliminated. A good example is closed loop/refrigeration processes where process gas treatment is costly.
- Maintainability: Some operators now have more experience with dry gas seals than with oil seals. This may compel end-users to retrofit a fleet at a specific plant or site to achieve consistent sealing technology throughout.Dry gas seals are supplied as cartridges by vendors, and the gas seal OEM usually performs their maintenance/refurbishment.These seven benefits may not be applicable to all situations, and it should be noted that wet seals to dry gas seals conversions are not straightforward. The following recommendations are offered to help make the retrofit project a success.
How to ensure a successful retrofit from wet seals to dry gas seals
Physical Integration: Integration of the dry gas seals in the original compressor head/cavity must be checked. The number and location of supply and vent ports (at least four ports are required on gas seals) should be reviewed. End-users should also consider inboard and outboard diameters; seal cartridge length; and the locking system of the gas seal to the compressor shaft.
In some instances, compressor shaft and compressor head rework are required. This should be assessed as soon as possible during the project to avoid project delays and cost overruns.
Seal Systems Study: While dry gas seals operation is usually not a concern during normal running conditions, transient conditions (start-up including first start, shutdown) and standby (pressurized and unpressurized) must be taken into account during the seal system design. In other words, a supply of dry and filtered seal gas at the right pressure must be ensured at all times.
The use of an alternate source of seal gas may be required during start-up, shutdown, and standby. If not available on site, end-users may consider supplying a conditioning skid. This can include a pre-filter, booster, and heater.
In any case, the best way to select the proper source of seal gas is to run a phase map analysis and make sure that a sufficient margin (20 C per API 614) to the dew point line (and hydrates formation line, if applicable) is maintained at all times in the gas seal panel and inside the gas seal.
On top of the suitability of seal gas, availability and suitability of secondary seal gas (when tandem gas seals with intermediate labyrinth are selected) and separation gas (usually nitrogen or air for separation barrier seals or labyrinth) must be checked.
Finally, a physical integration study of the gas seal panel must be performed, including space requirements and interconnecting piping/tubing to and from the compressor.
Rotor Dynamic Check: Retrofitting from wet seals to dry gas seals will affect rotor dynamic response since oil seals have better damping characteristics than dry gas seals. Performing a rotor dynamic analysis will confirm if amplification factor and logarithmic decrement are still acceptable with gas seals. In most cases, no further modification is required; however, there are some critical applications (long shaft, high speeds, etc.) where additional upgrades must be incorporated (damper bearings, hole pattern seals, etc.).
Training: Training should be standard practice. While dry gas seals usually require little monitoring, they are considered “black boxes.” There are a few indicators that can help assess the health of a gas seal.
Proper assembly and disassembly (in and from the compressor) is also of prime importance. Failing to do so may lead to premature dry gas seal failures.
Weighing the advantages of seal conversion
Dry gas seals have several advantages compared to conventional wet seals: higher reliability; safer operation; reduced emissions; lower operation and maintenance costs; and improved process gas quality. These advantages may help end-users justify an investment if an acceptable return on investment can be demonstrated.
However, as described in the second part of this article, a careful review of the system and its operating conditions is required. Provided all precautions are taken, dry gas seals may well be the most reliable mechanical seals currently available.
Raphaël Bridon began his career with Dresser-Rand in 1999 as a technical support engineer. He then moved to a Project Development Engineer position for reciprocating compressors before working as a Key Account Purchaser in the aeronautic business. He returned to Dresser-Rand in 2007 as the Manager for the Gas Seals and Bearings Business Unit. Mr. Bridon earned his master’s degree from Ecole Centrale Nantes (France).